Short channel amorphous In–Ga–Zn–O thin-film transistor arrays for ultra-high definition active matrix liquid crystal displays: Electrical properties and stability Soo Chang Kim a,b , Young Sun Kim a , Eric Kai-Hsiang Yu b , Jerzy Kanicki b,⇑ a YE Team, Samsung Display, Asan, Chungnam 336-741, South Korea b Solid-State Laboratory, Electrical Engineering & Computer Science, University of Michigan, Ann Arbor, MI 48109, USA article info Article history: Received 17 November 2014 Received in revised form 18 April 2015 Accepted 6 May 2015 Available online 5 June 2015 Keywords: In–Ga–Zn–O Thin-film transistor Active matrix liquid crystal display (AM-LCD) Short channel effect Tikhonov’s regularization AC bias-temperature stress stability (AC BTS) abstract The electrical properties and stability of ultra-high definition (UHD) amorphous In–Ga–Zn–O (a-IGZO) thin-film transistor (TFT) arrays with short channel (width/length = 12/3 lm) were examined. A-IGZO TFT arrays have a mobility of 6 cm 2 /V s, subthreshold swing (S.S.) of 0.34 V/decade, threshold voltage of 3.32 V, and drain current (I d ) on/off ratio of <10 9 with I off below 10 13 A. Overall these devices showed slightly different electrical characteristics as compared to the long channel devices; non-saturation of output curve at high drain-to-source voltage (V ds ), negative shift of threshold voltage with increasing V ds , and the mobility reduction at high gate voltage (V gs ) were observed. The second derivative method adopting Tikhonov’s regularization theory is suggested for the robust threshold voltage extraction. The temperature dependency of c-value was established after taking into consideration the impact of sour- ce/drain contact resistances. The AC bias-temperature stress was used to simulate the actual operation of active matrix liquid crystal displays (AM-LCDs). The threshold voltage shift had a dependency on the magnitude of drain bias stress, frequency, and duty cycle due to the impact ionization accelerated at high temperature. This study demonstrates that the short channel effects, source/drain contact resis- tances and impact ionization have to be taken into account during optimization of UHD AM-LCDs. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Oxide semiconductors such as an amorphous indium gallium zinc oxide (a-IGZO) have been intensively studied for past ten years especially for the application to next generation flat panel displays [1–3]. Superior characteristics of the a-IGZO thin film transistors (TFTs) as compared to the hydrogenated amorphous Si (a-Si:H) TFTs such as a high field effect mobility and trans- parency have increased their potential to become switching or driving transistors in next generation active matrix liquid crystal display (AM-LCD) or active matrix organic light emitting diode (AM-OLED), respectively. Today it has been already established that a-IGZO TFTs can be processed uniformly over large-size area at low temperature, and can use existing active-matrix arrays pro- duction infrastructure [4–6]. Recently, the high-end displays with the ultra-high definition (UHD > 4000  2000 pixels), large-area (>70 in.), and high frame rate (>240 Hz) are being introduced to realize displays with more realistic visual images [7]. To fabricate such displays, the dimension of TFTs, such as channel length and width, needs to be reduced to keep a high transmittance of pixel electrodes without any decrease of pixel aperture ratio and carrier mobility. Prior studies have focused on the electrical performance or electrical stability of TFTs with the relatively large channel width W (>100 lm) and length L (>10 lm) to avoid the short chan- nel and source/drain contact effects. Moreover, all reports pub- lished so far on a-IGZO TFTs have focused on a single TFT structures [8–12]. To our best knowledge, there is no report in open literature on electrical characteristics of the a-IGZO TFT arrays with a short channel length used in the UHD AM-LCDs. It is important to notice that the exact understanding of the funda- mental electrical properties of the a-IGZO TFT arrays with a UHD resolution is required to objectively compare the electrical behav- iors of the devices manufactured under different conditions. Recent studies of a-IGZO electrical bias-temperature stability (BTS) have employed a pulsed AC gate and/or drain bias simultaneously to reflect the typical driving scheme in AM-LCDs [13–18]. However, the unipolar pulsed gate stress (e.g. 0 V to positive or negative to 0 V pulse) has been often used with the source/drain grounded. Also the duty cycle, which is defined as the ratio of turn-on time http://dx.doi.org/10.1016/j.sse.2015.05.002 0038-1101/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 734 936 0964; fax: +1 734 615 2843. E-mail address: kanicki@umich.edu (J. Kanicki). Solid-State Electronics 111 (2015) 67–75 Contents lists available at ScienceDirect Solid-State Electronics journal homepage: www.elsevier.com/locate/sse